In the story, the boys encounter a wild piglet at the end of Chapter One.
The encounter with the piglet is indeed a significant moment in the chapter. It is the first time the boys are faced with the reality of their primal natures. Also, Jack's fierce demonstration of slamming the knife into the tree foreshadows his later acts of (real) violence in the novel.
In Chapter One, Ralph, Jack, and Simon explore the island. They are pleased that no one else inhabits the island and feel a sense of exhilaration at being its first occupants. The boys look for sources of food and survey the land before them. Eventually, they discover a plant they call a candle bush. The plant is aromatic, and the buds are green. Ralph comments that the buds look like candles. For his part, Jack thinks that the plant is useless because they can't use any part of it for food.
Eventually, the boys hear squealing and upon investigating, come across a wild piglet stuck in a tangle of creepers. Jack brandishes his knife and holds it upright, poised to strike. However, he doesn't kill the pig. Meanwhile, Ralph proclaims that one should always be inclined to "stick a pig." In his own defense, Jack argues that it is better to slit a pig's throat.
At this point, the other boys question why Jack didn't strike when he had the opportunity to do so. Embarrassed by their insinuation about his lack of masculine resolve, Jack slams his knife into a tree trunk. His message is clear: there will be no mercy for the pig next time.
Certainly, Jack's initial hesitation to kill is motivated by the habits of social convention. However, his aggressive proclamation about "next time" shows that refined behavior is nothing more than a facade. It can be consumed by primeval desperation when all vestiges of civilization have been removed. This is demonstrated when Jack later leads a team of "hunters" in the violent slaughter of a pig.